Polytetrafluoroethylene-silicone compositions



States Patent 2,934,515 Patented Apr. 26, 1960 POLYTETRAFLUOROETHYLENE-SILICONE COMPGSHTION S No Drawing. Application July 1, 1957 Serial No. 668,876

Claims. (Cl. 260-455) This invention relates to improved polytetrafluoroethylene compositions and articles manufactured therefrom.

Solid polytetrafluoroethylene has been widely accepted in industry because of its extreme inertness. It has withstood boiling solvents, including a variety of halogenated hydrocarbons, ketones, esters and alcohols without swelling or change in weight. Therefore, it has become popular as a material for gaskets where such solvents are present. Its tear strength in such a form ranges from 400 to 800 pounds per inch. As for tensile strength it being to draw at loads of 1500 to 2000 pounds per square inch. This material also has excellent dielectric properties. It retains these properties over a temperature range from 50 F. to over 500 F.

The main objection to solid polytetrafluoroethylene as a gasket material is its lack of resiliency. It is hard and stiff thereby requiring strong glands, flanges and bolts in order to compress a gasket sufliciently to effect a seal against the metal confining surfaces. Once a seal is effected, it is difficult to maintain it for two reasons. First, a gasket once distorted restores to its original shape only slowly whereby the gaskets leak when conditions change. This presents a major problem in batch operation or in any situation where variation in temperature changes the pressure on the gasket through expansion and contraction of the flanges and bolts. Second, a polytetrafluoroethyene gasket exhibits a high degree of cold-flow" even for moderate loads. Therefore, the gasket and flange joint must be constantly readjusted to maintain a good seal. An improvement in resiliency would negate the eflect of cold-flow." Furthermore, a suitable composition might bind the polytetrafluoroethylene sufficiently to eliminate cold-flow.

Up to this time these short-comings have been compensated for by mechanical means such as the use of spring-loaded bolts in gasketed joints. However, there has been no solution to these problems.

Therefore, it is the object of this invention to prepare solid polytetrafluoroethylene compositions which have improved resiliency without sacrificing the solvent resistance, thermal stability and dielectric strength of solid polytetrafluoroethylene by itself.

This invention relates to a mixture of (1) solid polytetrafluoroethylene and 20 to 80 parts by weight per 100 parts of (1) of (2) an organopolysiloxane gum consisting essentially of units of the formula where each R is hydrogen or methyl radicals, each R is a monovalent perfiuoroalkyl radical of from one to ten inclusive carbon atoms and each n has a value of from 1 to 4 inclusive. Furthermore,siloxane (2) can also contain up to mol percent of any combination of siloxanes of the formula R" SiO where R is phenyl, vinyl or methyl.

The term consisting essentially of" indicates that the siloxane gum is made up primarily of diorganosiloxane units, but a limited number of triorganosiloxane units, e.g.

CH3 (IJH CFsCHzCHz-Si-Q.s or CHs-EiiOa H: CH3

and monoorganosiloxane units, e.g. CF CH CH SiO or C H SiO can be present without putting the resulting gums outside the scope of this invention.

As shown above in siloxane (2), R can be hydrogen or the methyl radical. R can be any monovalent periiuoroalkyl radical of from 1 to 10 carbon atoms, such as trifiuoromethyl, pentafiuoroethyl, heptafluoropropyl, perfluoroheptyl or perfluorodecyl radicals. n can have a value of from 1 to 4 thereby including within the scope bf this invention such radical as CF CH CH CaF1 CHaCHCHrand CF: H(CHCH2)4 CaFs (IJF: ([1213 CzFgCHzCHgGHOHw CHaCHCHzCHCHz- ($Fa 02F OF! H CHCH: 8- CsF1CHeCH2CHCHzOHCHP and (CF: H /HCHzr 4 If less than 20 parts by weight of gum (2) per 100 parts of polytetrafluoroethylene (1) is employed, the eflect of the gum is lost. If more than parts by Weight of gum (2) per parts of polytetrafluoroethylene (1) is employed, tensile strength is markedly decreased.

The siloxane gum (2) of this invention can be either a homopolymer or copolymer and is prepared by polymerizing cyclic trisiloxanes of the formula R CH3 [Bowl] to where R, n and R are as above defined. The best method of doing this is described in the copending application of Oscar K. Johannson, Serial No. 594,107, filed June 27, 1956. In general, the polymerization of the cyclic trisiloxane is best carried out in the presence of an alkaline catalyst such as alkali metal hydroxides or the siloxane salts of alkali metal hydroxides at temperatures ranging from 50 to C. The preparation of cyclic trisiloxanes from chlorosilanes is described in the copending application of Ogden R. Pierce and George W. Holbrook, Serial No. 594,108, filed June 27, 1956. These methods are operative where R is hydrogen or the methyl radical and where n has any value from 1 to 4. Both of these copending applications are hereby made a part of this specification by reference.

The corresponding chlorosilanes are disclosed in the following copending applications:

3m CH: OHaCHCHzS iCIz Serial No. 608,667, filed September 10, 1956, by Paul Tarrant and George W. Dyckes;

Serial No; 608,668, filed September 10, 1956, by Paul Tarrant, now abandoned; and

3 Serial No. 644,479, filed March 7, 1957, by Ogden R. Pierce, now Patent 2,894,969. These applications are incorporated into this specification by this reference.

In general, fluorinated alkenes of the formula RI R=CH2 are reacted with CH: HS iCl:

under proper conditions to form either the simple adduct or the telomeric adducts.

As stated above, siloxane (2) can also contain up to mol percent of any combination of phenylmethylsiloxane units, dimethylsiloxane units, diphenylsiloxane units, phenylvinylsiloxane units, divinylsiloxane units and methylvinylsiloxane units. These are best included by mixing the corresponding cyclic trisiloxanes with the abovementioned cyclic trisiloxanes and performing an alkaline copolymerization as described above.

The compositions of this invention are best mixed by milling the gum on a standard two roll rubber compounding mill and adding dry particles of solid polytetrafiuoroethylene, which is readily available on the market. The shearing action of the mill will cause the polytetrafiuoroethylene particles to deform and elongate, forming fibers in the gum. Subsequent milling in a cross direction will produce a solid matting of fibers held by the gum so that tensile strength and tear strength will be good in all directions. Alternatively, the polytetrafiuoroethylene can be added in a dispersion in any carrying agent, such as water, which does not adversely alfect the siloxane gum.

The milled compositions can be vulcanized by any of the known methods for vulcanizing organosiloxane gums. One of the preferred methods comprises heating the compound with an organic peroxide such as benzoyl peroxide, tert-butyl perbenzoate, dicumyl peroxide, chlorobenzoyl peroxides and tert-butyl peracetate. Preferably these peroxides are employed in amount from 0.1 to 10% by weight based on the weight of the gum.

Additives such as pigments and dyes can be added. However, any additive which acts as a filler will tend to make the polytetrafiuoroethylene more incompatible with the gum thereby increasing the minimum amount of gum which can be used. The common rubber fillers will generally defeat this invention by decreasing the millability of the unvulcanized compound thereby prohibiting an even mixture and by increasing the stiffness of the finished product.

The resulting composition has the solvent-resistance, thermal stability and dielectric properties of polytetrafluoroethylene while having much improved resilience. This material is therefore especially useful as gaskets.

Furthermore, the compositions of this invention require lower molding pressure and temperature than is necessary with polytetrafiuoroethylene itself.

The following examples are illustrative and are not intended to limit this invention which is properly set out in the claims.

Example 1 Three compositions were prepared by milling on a two roll rubber-compounding mill. Composition I consisted of 100 parts by weight of polytetrafiuoroethylene, 80 parts by weight of a gum (A) consisting of 99.5 mol percent 3,3,3-trifluoropropylmethylsiloxane and 0.5 mol percent methylvinylsiloxane and 1.0 part by weight of bis- (2,4-dichloro)benzoyl peroxide. Composition II consisted of 100 parts by Weight of polytetrafluoroethylene, 40 parts by weight of gum (A) and .4 part by weight of bis- (2,4-dichloro)benzoyl peroxide. Composition Ill consisted of 100 parts by weight of polytetrafiuoroethylene, 33.3 parts by weight of gum (A) and .5 part of bis-(2,4- dichloro)-ben zoyl peroxide. The bis-(2,4-dichloro)benzoyl peroxide was added as a 40% by weight dispersion in dimethylpolysiloxane fluid.

Each composition was filled until uniform, and samples were checked for durometer hardness, tensile strength (pounds per square inch), elongation (percent) and tear strength (pounds per inch). Subsequently, each composition was cured for 24 hours at 150 C. and samples were checked as above. The results were as follows:

As Milled After 24 hr. at 150 0. Comp.

D T I E Tear D T E Tear The elongation of polytetrafluoroethylene is approximately zero percent. Thus, these compositions have satisfactory characteristics to make excellent gaskets. These compositions are extrudable whereas polytetrafluoroethyle'ne is not.

Example 2 When CF: CE: I 'l I, H CHOH: -s10 l units and (FF: (7H1 CHsCHCHzSlO units are each substituted for the (311: CFECHQCHZSiO units of gum (A) in Example 1, similar excellent compositions are produced.

Example 3 When a copolymer consisting of mol percent of C'IFIBOHZCHZSlO units, 6 mol percent dimethylsiloxane units, 3 mol percent methylvinylsiloxane units and 1 mol percent phenylmethylsiloxane units is substituted for gum (A) in Example l, similar suitable gasket materials are produced.

units are substituted for gum (A) in Example 1, similar compositions are produced.

That which is claimed is:

1. A composition of matter comprising a mixture of (1) solid polytetrafiuoroethylene and 20 to 80 parts by weight per parts of 1) of (2) an organopolysilox ane gum selected from the group consisting of polymers in which all of the recurring units are of the formula where each R is selected from the group consisting of hydrogen and methyl radicals, each R is a monovalent perfiuoroalkyl radical of from 1 to 10 inclusive carbon atoms and n has a value from 1 to 4 inclusive and copolymers containing at least 90 mol percent of the aforesaid siloxane units and up to mol percent siloxane units of the formula RI where R" is selected from the group consisting of methyl, phenyl and vinyl radicals.

2. The composition of claim 1 in which all of the recurring units of organopoly siloxane (2) are i i i E H H 3. A composition of claim 1 in which siloxane (2) is a copolymer containing at least 90 mol percent of the recurring units H H o and up to 10 mol percent of the recurring units HHCHs 4. A composition of matter comprising a mixture of (1) solid polytetrafluoroethylene and 20 to 80 parts by weight per 100 parts of 1) of (2) an organopolysiloxane gum selected from the group consisting of polymers in which all of the recurring units are of the formula R1 H CH3 where each R is selected from the group consisting of hydrogen and methyl radicals, each R is a monovalent perfluoroalkyl radical of from 1 to 10 inclusive carbon atoms and n has a value from 1 to 4 inclusive and copolymers containing at least mol percent of the aforesaid siloxane units and up to 10 mol percent siloxane units of the formula where R" is selected from the group consisting of methyl, phenyl and vinyl radicals, and (3) an organic peroxide curing agent for (2).

5. A composition comprising a mixture of (1) solid polytetrafluoroethylene and 20 to 80 parts by weight per parts of 1) of (2) a cured, solid, elastic organopolysiloxane in which all of the recurring units are F H H CH: r H)- l t l 1 References Cited in the file of this patent UNITED STATES PATENTS 2,710,290 Safford et a1 June 7, 1955 2,719,833 Vincent et al. Oct. 4, 1955 OTHER REFERENCES 4 UNITED STATES PATENT CERTIFICATE, OF CORRECTION Patent N 2,934,515 April 26, 1960 George M. Konkle et a1.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 25, for "being read begins column 4, lines 67 to 69, the formula should appear as shown below instead of as in the patent:

column 5, lines 3 to 6, the formula should appear as shown below instead of as in the patent:

Si-O- I R// same column 5, lines 33 to 36, the formula should appear as shown below instead of as in the patent:

Signed and sealed this 27th day of September 1960.

(SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Attesting Officer Commissioner of Patents 

1. A COMPOSITION OF MATTER COMPRISING A MIXTURE OF (1) SOLID POLYTETRAFLUOROETHYLENE AND 20 TO 80 PARTS BY WEIGHT PER 100 PARTS OF (1) OF (2) AN ORGANOPOLYSILOXANE GUM SELECTED FROM THE GROUP CONSISTING OF POLYMERS IN WHICH ALL OF THE RECURRING UNITS ARE OF THE FORMULA 